The present invention relates to a liquid crystalline compound and a liquid crystal composition. In more detail, the invention relates to a fluoroalkoxybenzene derivative having an ether linkage and being preferable as a component of liquid crystal composition, a liquid crystal composition comprising the derivative, and a liquid crystal display element fabricated by using the liquid crystal composition.
Liquid crystal display elements utilize optical anisotropy and dielectric anisotropy of liquid crystalline compounds. They are divided into modes such as twisted nematic (TN), super twisted nematic (STN), dynamic scattering (DS), guest-host (GH) and DAP type mode or so depending on their display modes on the electric optical effects, and also divided into driving modes such as static, time sharing addressing, active matrix and two-frequency addressing scheme.
Recently, particularly high quality display elements are required, and needs for display elements of active matrix types exemplified by a thin film transistor (TFT) type is increased. Furthermore, in order to improve response time against a change of an electric field or in order to lower a driving voltage, there are particularly required liquid crystal materials having higher dielectric anisotropy values (xcex94xcex5) and lower viscosity values. Liquid crystal materials used in said liquid crystal, display elements are required to exhibit a liquid crystal phase at temperatures in a wide range and to be stable against heat, light, moisture, air, electric field and electromagnetic radiation or so. However, no compounds are found at present which satisfy such requirements by a single compound, and thus it is a current situation that a plurality, often as much of 20 or more, of liquid crystalline compounds are mixed and used. In particular, since there are recently many cases that liquid crystal display elements are used under such severe conditions as a quite low temperature or so, improvement in miscibility at a low temperature is also required.
Since TFT type liquid crystal display element needs a holding of electric charge stored between electrodes of pixels during a flame period, liquid crystal materials therefor are required to have a particularly high voltage holding ratio. Liquid crystalline compounds having fluorine atoms in their molecules have been used hitherto as liquid crystal materials which satisfy such requirements and have relatively high xcex94xcex5.
For example, fluoroalkoxybenzene derivatives shown by the formulae (10) to (18) 
wherein R denotes an alkyl group and Rf denotes a fluoroalkyl group, are now known.
However, compounds shown by the general formulae (10) or (11) which are disclosed in WO 94/26838 or DE 19528085 A1 have no or narrow liquid crystal temperature range, and furthermore their electric anisotropy values can never be said high enough.
As to compounds shown by the general formula (12) which are disclosed in Laid-open Japanese Patent Publication No. Hei 7-165656, their dielectric anisotropy values can also never be said high enough. Furthermore, compounds shown by the general formula (14) are disclosed in DE 4027840 A1, but there is no description about their electric characteristics. Furthermore, compounds shown by the general formulae (13), (15), (16) and (18) are disclosed in DE 4218614 A1, EP 640578 A1, DE 4223501 A1, DE 4222371 A1, WO 9213928 A1, DE 4142519 A1 and DE 4027840 A1 or so. But there is no description about their physical properties, so that it can never be said that the invention is disclosed.
In addition, compounds shown by (17) are disclosed without any information about electric characteristics in DE 4301700 A1, but said compounds are considered to have no sufficient xcex94xcex5 value because dipolar moments thereof are offset by fluoroalkoxy groups at both terminals of the compounds.
We the inventors have studied eagerly to solve the above-mentioned problems and found a novel compound having improved characteristics compared to the known liquid crystalline compounds, and have completed the present invention.
As is clear from the above-mentioned description, an object of the present invention is to provide a liquid crystalline compound of fluoroalkoxybenzene derivatives having ether bond, having in particular large xcex94xcex5 value, being excellent in miscibility with other liquid crystalline compounds, having a low viscosity and being stable chemically and physically, as well as a liquid crystal composition, and a liquid crystal display element comprising the compound.
Inventions to be claimed in the present application are as follows.
(1) A fluoroalkoxybenzene derivative expressed by the general formula (1) 
wherein A1, A2 and A3 each independently denote trans-1,4-cyclohexylene,1,4-phenylene in which one or more hydrogen atoms may optionally be substituted with a fluorine atom(s), 1,3-dioxane-2,5-diyl, pyrimidine-2,5-diyl or 1-sila-1,4-cyclohexylene; B1, B2 and B3 each independently denote a single bond, 1,2-ethylene, 1,2-ethenylene, 1,2-ethynylene, oxymethylene, methyleneoxy, carbonyloxy or 1,4-butylene group; R denotes an alkyl group having 1 to 15 carbon atoms optionally substitutable with a halogen atom(s), wherein one or more non-adjacent ethylene groups may be replaced by 1,2-ethenylene group(s); Rf denotes a fluoroalkyl group having 1 to 3 carbon atoms substitutable with two or more fluorine atoms; k, m, and n each independently denote 0 or 1, with the proviso that when k=0 is a case, then one of B1, B2 and B3 is a methyleneoxy or oxymethylene bond, and that when m+nxe2x89xa71 is a case, then A1 or/and A2 and A3 are not 1,4-phenylene at the same time, and further when A1 or/and A2 and A3 are trans-1, 4-cyclohexylene or 1,4-phenylene and B1 or/and B2 and B3 are all single bonds, then Rf is never C2F5, CH2CF3 or CH2CF2CF3, and that when m+n=1 is a case, A1 or A2 and A3 are trans-1, 4-cyclohexylene at the same time, B1 or B2 is a single bond and B3 is 1,2-ethylene, then Rf is never CH2CF2CF3.
(2) A compound recited in paragraph (1) above wherein k=1.
(3) A compound recited in paragraph (1) above wherein one of B1, B2 or B3 is a methyleneoxy or oxymethylene bond.
(4) A compound recited in paragraph (2) above wherein m=0, n=1, A2 and A3 are 1, 4-cyclohexylene, and B2 and B3 are single bonds.
(5) A compound recited in paragraph (2) above wherein m=0, n=1, A2 is 1, 4-cyclohexylene, A3 is 1, 4-cyclohexylene optionally substitutable with a fluorine atom(s), and B2 and B3 are both single bonds.
(6) A compound recited in paragraph (2) above wherein m+n=1, A1 or A2 and A3 are both trans-1, 4-cyclohexylenes, B1 or B2 is 1, 2-ethylene, and B3 is a single bond.
(7) A compound recited in paragraph (2) above wherein m+n=1, A1 or A2 and A3 are both trans-1, 4-cyclohexylenes, B1 or B2 is a single bond, and B3 is 1,2-ethylene.
(8) A compound recited in paragraph (2) above wherein m=n=1, and A1 and A2 are both trans-1, 4-cyclohexylenes.
(9) A compound recited in paragraph (8) above wherein m=n=1, A1 and A2 are both trans-1, 4-cyclohexylenes, A3 is 1, 4-phenylene one or two hydrogen atoms of which may optionally be substituted with a fluorine atom(s), B1 and B3 are single bonds, and B2 is 1,2-ethylene.
(10) A liquid crystal composition comprising at least one liquid crystalline compound recited in any one of paragraphs (1) to (9).
(11) A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs (1) to (9), and as a second component, at least one compound selected from the group of compounds expressed by any one of general formulae (2), (3), and (4) 
wherein R3 denotes an alkyl group having 1 to 10 carbon atoms in which alkyl group any optional non-adjacent ethylene group may be replaced by an oxygen atom or xe2x80x94CHxe2x95x90CHxe2x80x94, and any optional hydrogen atom may be substituted with a fluorine atom; Y1 denotes a fluorine atom, a chlorine atom, OCF3, OCF2 H, CF3, CF2H, CFH2, OCF2CF2H or OCF2CFHCF3; L1 and L2 each independently denote a hydrogen atom or a fluorine atom; Z1 and Z2 each independently denote 1,2-ethylene group, 1,4-butylene group, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94, or a single bond; ring B denotes trans-1,4-cyclohexylene, 1,3-dioxane-2,5-diyl or 1,4-phenylene any hydrogen atom of which may be substituted with a fluorine atom; and ring C denotes trans-1,4-cyclohexylene or 1,4-phenylene any hydrogen atom of which may be substituted with a fluorine atom.
(12) A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs (1) to (9), and as a second component, at least one compound selected from a group of compounds expressed by any one general formulae (5) and (6) 
wherein R4 and R5 each independently denote an alkyl group having 1 to 10 carbon atoms in which alkyl group any optional non-adjacent methylene group may be replaced by an oxygen atom or xe2x80x94CHxe2x95x90CHxe2x80x94 and any optional hydrogen atom of which may be substituted with a fluorine atom; Y2 denotes xe2x80x94CN group or xe2x80x94Cxe2x89xa1Cxe2x80x94CN group; ring E denotes trans-1,4-cyclohexylene, 1,4-phenylene, 1,3-dioxane-2,5-diyl or pyrimidine-2,5-diyl; ring G denotes trans-1,4-cyclohexylene, 1,4-phenylene any hydrogen atom of which may be substituted with a fluorine atom or pyrimidine-2, 5-diyl; ring H denotes trans-1, 4-cyclohexylene or 1,4-phenylene; Z3 denotes 1,2-ethylene group, xe2x80x94COOxe2x80x94 or a single bond; L3, L4 and L5 each independently denote a hydrogen atom or a fluorine atom, and b, c and d each independently denote 0 or 1.
(13) A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs (1) to (9), as a second component, at least one compound selected from the group of compounds expressed by any one of general formulae (2), (3), and (4) described above, and as a third component, at least one compound selected from a group of compounds expressed by any one of the general formulae (7), (8) and (9) 
wherein R6 and R7 each independently denote an alkyl group having 1 to 10 carbon atoms in which alkyl group any optional non-adjacent methylene group may be replaced by an oxygen atom or xe2x80x94CHxe2x95x90CHxe2x80x94 and any optional hydrogen atom of which may be substituted with a fluorine atom; I, J and K each independently denote trans-1,4- cyclohexylene, pyrimidine-2,5-diyl or 1,4-phenylene any hydrogen atom of which may be substituted with a fluorine atom; and Z4 and Z5 each independently denote xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond.
(14) A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs (1) to (9), as a second component, at least one compound selected from the group of compounds expressed by any one of general formulae (5) and (6) described above, and as a third component, at least one compound selected from a group of compounds expressed by any one of the general formulae (7), (8) and (9) described above.
(15) A liquid crystal composition comprising, as a first component, at least one liquid crystalline compound recited in any one of paragraphs (1) to (9), as a second component, at least one compound selected from the group of compounds expressed by any one of general formulae (2), (3), and (4) described above, as a third component, at least one compound selected from a group of compounds expressed by any one of the general formulae (5) and (6) described above, and as a fourth component, at least one compound selected from a group of compounds expressed by any one of the general formulae (7), (8) and (9) described above.
(16) A liquid crystal composition recited in any one of paragraph (10) to (15) wherein the liquid crystal composition further comprises one or more optically active compounds.
(17) A liquid crystal display element fabricated by using a liquid crystal composition recited in any one of paragraphs (10) to (15)
Although the liquid crystalline compounds of the present invention are expressed by the general formula (1) as described above, the compounds expressed by the following formulae (1-1) to (1-71) can particularly be mentioned as preferable examples amongst of them. 
In the above-mentioned formulae, R denotes an alkyl group, W denotes methyleneoxy or oxymethylene group, and Rf denotes a fluoroalkyl group having 1 to 3 carbon atoms in which two or more fluorine atoms are substituted.
Any of these compounds of the present invention have superior characteristics such as exhibiting particularly high dielectric anisotropy, high voltage holding ratio, low viscosity and good miscibility at a low temperature. In particular, two-ring or three-ring system compounds expressed by the formulae (1-1) to (1-26) exhibit high dielectric anisotropy and good miscibility at a low temperature. Therefore, by using liquid crystal compositions comprising these compounds, liquid crystal cells to be driven at a low voltage can be manufactured. Furthermore, four-ring system compounds expressed by the formulae (1-27) to (1-71) exhibit high dielectric anisotropy, and have eminently high clear point while have relatively low viscosity.
Therefore, by using liquid crystal compositions comprising these compounds, liquid crystal cells with wide display temperature range and being driven at low voltage can be manufactured.
Liquid crystalline compounds of the present invention expressed by the general formula (1) do not always exhibit liquid crystal phase. But they are effective when they are mixed with other liquid crystalline compounds, because any of them has a good miscibility with other liquid crystalline compounds and can give compositions without eminently lowered nematic phase temperature or reduced range thereof. Therefore, any of the liquid crystalline compounds expressed by the general formula (1) having such superior optical characteristics as described above can be an useful constitutional component of a liquid crystal composition, even though it does not exhibit a liquid crystal phase by itself.
Liquid crystal compositions of the present invention comprise, as the first component, at least one liquid crystalline compound expressed by the general formula (1).
Its content is preferably 0.1 to 99.9% by weight based on the amount of liquid crystal composition for developing excellent characteristics.
While the liquid crystal composition of the present invention may comprise only the first component described above, the composition in which at least one compound selected from the group consisting of the compound expressed by one of the general formulae (2), (3) and (4) described above (hereinafter referred to as second component A) and/or at least one compound selected from the group consisting of the compound expressed by the general formulae (5) or (6) described above hereinafter referred to as second component B) are mixed as second component in addition to the first component, or the compositions in which at least one compound selected from the group consisting of the compound expressed by one of the general (7), (8), and (9) described above are further mixed as a third component in addition to the first and second components are preferable. Besides, an optically active compound as another component, and a known compound may be mixed for the purpose of adjusting threshold voltage, temperature range of liquid crystal phase, optical anisotropy value (xcex94n), xcex94xcex5 and a viscosity.
Among the second component A described above, the compounds expressed by one of the following formulae (2-1) to (2-9) can be mentioned as preferable examples of the ones included in the general formula (2), the compounds of one of the following formulae (3-1) to (3-69) can be mentioned as preferable examples of the ones included in the general formula (3), and the compounds of one of the following formulae (4-1) to (4-24) can be mentioned as preferable examples of the ones included in the general formula as preferable examples of compounds included in the general formula (4), respectively. 
wherein R3 and Y1 have the same meanings as described above.
Any of the compounds expressed by one of the general formulae (2) to (4) exhibit a positive xcex94xcex5, are excellent in thermal stability and chemical stability, and are indispensable when liquid crystal composition for TFT (AM-LCD) of which a high reliability such as a high voltage holding ratio (large specific resistance) is required are produced.
While the amount of the compound to be used is suitably in the range of 1 to 99.9% by weight based on the total amount of liquid crystal when liquid crystal composition for TFT are produced, it is preferably 10 to 97% by weight and more desirably 40 to 95% by weight. In this case, liquid crystal composition may further comprise a compound expressed by one of the general formulae (7) to (9) for the purpose of adjusting viscosity.
While the compounds expressed by one of the general formulae (2) to (4) described above can be used when liquid crystal compositions for STN display mode or a TN mode are produced, the amount of the compound to be used is preferably less than 50% by weight based on the total amount of liquid crystal composition since this compound is small in its effect of lowering threshold voltage of the liquid crystal composition.
Next, among the second component B, the compounds of one of the formulae (5-1) to (5-40) can be mentioned as preferable examples of the ones included in the general formula (5), and the compounds of one of the formulae (6-1) to (6-3) can be mentioned as preferable examples of the ones included in the general formula (6), respectively. 
wherein R4, Y2 and R5 have the same meanings as described above.
Any of the compounds expressed by the general formulae (5) or (6) has a positive and large xcex94xcex5 value, and is used particularly for the purpose of lowering threshold voltage of liquid crystal compositions.
Also, the compounds are used for the purposes of improving the steepness of liquid crystal compositions for STN display mode or TN display mode including for the purposes of adjusting xcex94n and raising clearing point of liquid crystal compositions, and thus are indispensable particularly when liquid crystal compositions for STN display mode or TN display mode are produced.
Whereas the compounds can lower threshold voltage of liquid crystal compositions as their amount used is increased, the use of the compounds brings about increase of the viscosity.
Accordingly, it is advantageous to use the compounds in a large amount for driving display element at a lower so far as the viscosity of liquid crystal compositions satisfies required characteristics.
Under such circumstances, the amount of the compounds to be used is suitably in a range of 0.1 to 99.9% by weight, preferably 10 to 97% by weight, and more preferably 40 to 95% by weight, based on the total amount of liquid crystal composition when liquid crystal compositions for STN display mode or TN display mode are produced.
Among the third component described above, the compounds of one of the formulae (7-1) to (7-11) can be mentioned as preferable examples of the ones included in the general formula (7), the compounds of one of the formulae (8-1) to (8-18) can be mentioned as preferable examples of the ones included in the general formula (8), and compounds of one of the formulae (9-1) to (9-6) can be mentioned as preferable examples of compounds included in the general formula (9). 
wherein R6 and R7have the same meanings as described above.
Any of the compounds expressed by one of the general formulae (7) to (9) has a small absolute value of xcex94xcex5. Among them, the compounds of the general formula (7) are used for the purpose of adjusting viscosity or adjusting xcex94n of liquid crystal composition, and the compounds of the general formulae (8) and (9) are used for the purposes of widening nematic range by such a way of raising clearing point or adjusting xcex94n.
Whereas these compounds raise threshold voltage of the liquid crystal compositions as their amount used is increased, the use of compounds reduces the viscosity. Accordingly, it is desirable to use the compounds in a larger amount so far as the threshold voltage of liquid crystal compositions satisfies required values.
From such circumstances, the used amount of the compounds to be used is suitably less than 40% by weight and preferably less than 35% by weight based on the total amount of liquid crystal composition when liquid crystal composition for a TFT display mode are produced. On the other hand, when liquid crystal compositions for STN display mode or TN display mode are produced, the amount of use described above is suitably less than 70% by weight and preferably less than 60% by weight.
Among other components, an optically active compound is usually added to the liquid crystal compositions of the present invention for the purpose of inducing helical structure of liquid crystal compositions to adjust required twist angle and to prevent reverse twist, with the exception of specific cases, for instance, the case of liquid crystal compositions for OCB (Optically Compensated Birefringence) mode. While the optically active compound is widely selected from known compounds so far as the purposes described above can be achieved, the optically active compounds expressed by one of the following formulae (Op-1) to (Op-8) can preferably be mentioned. 
Pitch length of the twist in liquid crystal compositions is adjusted by adding these optically active compounds. The twist pitch length is preferably adjusted in the range of 40 to 200 xcexcm in the case of liquid crystal compositions for TFT or TN mode, preferably adjusted in the range of 60 to 20 xcexcm in the case of the liquid crystal compositions for STN mode, and preferably adjusted in the range of 1.5 to 4 xcexcm in the case of liquid crystal compositions for bistable TN mode, respectively. Further, in such cases, two or more kind of optically active compounds may be added for the purpose of adjusting the dependency of pitch length on temperature.
Liquid crystal compositions of the present invention can be prepared by methods that are conventional by themselves. For instance, the compositions are prepared by a method in which various components are dissolved in one another at a high temperature.
Further, the liquid crystal compositions of the present invention can be used as ones for a guest-host (GH) mode by adding a dichroic dye such as merocyanine type, styryl type, azo type, azomethine type, azoxy type, quinophthalone type, anthraquinone type or tetrazine type. Alternatively, the liquid crystal compositions may be used as NCAP which is prepared by the microencapsulation of a nematic liquid crystal, or as liquid crystal compositions for polymer dispersed liquid crystal display elements (PDLCD) represented by polymer network liquid crystal display elements (PNLCD) prepared by forming a polymer of three-dimensional reticulated structure in a liquid crystal. Still further, the liquid crystal compositions of the present invention can be used as ones for electrically controlled birefringence (ECB) mode or dynamic scattering (DS) mode.
As of nematic liquid crystal compositions comprising the compounds of and prepared by the present invention as described above are mentioned as Examples below.
In each of the following Composition Examples, compounds are designated by making the groups shown in each of columns of left side terminal group, bonding group, ring structure, and right side terminal group correspond to the symbols shown in the columns of symbol according to the definition shown in Table 1 below. 
Compound No. appended to the compounds of the present invention in the following Composition Examples means that the compounds are the same as those shown in Examples described below and having the same appended Compound No.; and the content of compounds means % by weight unless otherwise specified. Further, data of characteristics of compositions in Composition Examples are indicated by TNI (phase transition temperature of nematic phase-isotropic liquid, or clearing point), xcex7(viscosity, determined at a temperature of 20.0xc2x0 C.), xcex94n (optical anisotropy value, determined at a temperature of 25.0xc2x0 C.), Vth (threshold voltage, determined at a temperature of 25.0xc2x0 C.) and P (pitch, determined at a temperature of 25.0xc2x0 C.). Abbreviations shown herein are defined as table 1. [Table 1]